FTN Needle Seal Readiness test

I was wondering if someone could explain this diagnostic to me? Typically, a failure in this test seems to indicate a leak in the needle and seat flow path. I'll replace the needle seat or correct a leak between the needle seat housing and inject valve, and the issue is corrected. Lately, I'm finding that the diagnostic fails with a negative number. For example, flow of 0.4mL/min with a steady back pressure of 6700psi, and the needle seal readiness test gives a pressure difference between the two flow paths of only 5 psi. But because it's negative 5 psi the test still fails. If I inspect the sample needle, stationary tube, needle seat, and housing assembly, I can find nothing wrong. No signs of a leak or blockage. The only variable I can't check is the seal force sensor, but the needle seal calibration works flawlessly. If I ignore the test result, and continue my run, the system seems to function correctly. But occassionally there are random seal force errors. I don't like ignoring ACQUITY diagnostics because I have yet to find one that is not meaningful. Help? Thanks!Eric

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Answers

The Needle Seal Readiness test simple compares the pressure with the needle, needle seal and stationary side tube in the flow path to the pressure with these parts OUT of the flow path (injector valve in the Inject position vsLoad postion). A passing result is when this difference is more than 100 psi and failing if it is not. So the actual test starts in with the valve in the inject position, monitors the pressure, then switches the valve to the load position. At this point the pressure should drop due to the cange in backpressure from removing these tubes from the path.s

I can't remember if the test reports this change of pressure as a positive or negative number, but the difference HAS to be at least 100 psi.

In Eric's case, PInject - PLoad = - 5 psi which is < 100 so the test fails. Even if the value were +5, a Failing result would be obtained

Eric, assuming that the result can be believed, it is telling you that there is a leak. The leak is somewhere in the flow path of the needle i.e, valve port # 5 to valve port # 4 to the needle to the needle seal to the tubing connecting the injection port to valve port #1 to valve port #6.

Just as an aside, the system I made these observations on was configured as an ACQUITY 2D System. I observed that the System Pressure that is monitored during this test is the pressure measured by Pump 1. So, if you have a 2D system, make sure that you perform this test while Pump 1 is flowing through the injector. Otherwise, you will obtain erroneous results.

Thank you. This clears things up for me. I had a assumed that a large difference between PLoad and PInject indicated a leak, and that there would be additional volume/backpressure from Pinject which would be accounted for,, so the negative number did not make sense to me because it seemed to be absolute. Thank you again.

I'm afraid Marc and I did not accurately characterize the test during our earlier discussion.

I have since learned that there is no minimum value that determines a Passing result. The value simply must be positive, i.e., PInject - PLoad > 0.

The magnitude of the pressure difference between PInject and PLoad is dependent on many things, including PLoad (which of course depends on system flow rate), solvent viscosity, presence or absence of extension loops. Therefore, it would be difficult to set Pass/Fail criteria based on the magnitude of the pressure differential. You observed a passing result with a magnitude of 5 psi. I have observed passing results with magnitudes of 81, 189, 125 and 532 psi.

However, if there is no leak at the needle seal (or other parts of the "valve - needle - injection port - valve" flow path) we can say with certainty that PInject - PLoad will be a positive number.

No problem. I really appreciate your help with this. Everyone here looks to me for confirmation that a given system is performing correctly. Precision and Linearity injections don't always tell the entire story of a system's condition. Understand the logic and workings of the ACQUITY diagnostics helps me to fill in that gap so that I can confidently say whether or not a system is working as intended. Thanks again!